Ia-positive Cells Research Articles

Splenocytes and bone marrow cells from T-cell deficient Nu/Nu mice secrete interleukin 3 activity after stimulation in vitro

We show here that the combination of Concanavalin A (Con A), phorbol myristate acetate (PMA), and Ionomycin (Iono) reproducibly stimulated splenocytes from Nu/Nu mice and bone marrow cells from both normal and Nu/Nu mice to secrete interleukin 3 (IL-3) in vitro. IL-3 was measured by its property of supporting the growth of four different clones known to grow only in IL-3. None of the agents indicated above nor several other types of stimuli tested could induce the cells to secrete IL-3 activity. IL-3 activity from induced cells of either tissue was detected after 24 hr of culture, peaked at 48 hr and either declined by 72–96 hr of culture (bone marrow cells) or remained relatively constant through the 4-day culture period (splenocytes). The cells participating in the production of IL-3 activity in Nu/Nu spleen were THY1 +, L3T4 −, LyT2 −, B-220 −, J11d −, Ia −, and those in the marrow from either normal or Nu/Nu mice were THY1 +, J11d +, L3T4 −, LyT2 −, B-220 −, Ia −. Finally, we present evidence that Ia-positive cells negatively regulate the production of IL-3 activity by both splenocytes and marrow cells. We conclude that Nu/Nu splenocytes and bone marrow cells from both normal and Nu/Nu mice can secrete IL-3 activity after proper stimulation in vitro and that such property is negatively regulated by Ia-positive cells.

Phenotypic in situ characterization of lymphocytes in mild alcoholic liver disease.

The subsets and expression of activation markers of inflammatory mononuclear cells in mild alcoholic liver disease were studied using monoclonal antibodies and an avidin-biotin-peroxidase complex (ABC) method. There were few B cells and monocytes in situ. Most of the mononuclear cells were of T cell origin, T4 and T8 positive cells being evenly distributed in a ratio of 2 to 1. This contrasts with the present and earlier findings on autoimmune liver diseases. The frequency of Ia (HLA-DR) positive cells varied between 15 and 30% of all mononuclear cells, which indicates activation of the local T cells. This implies that T cells in alcoholic fatty liver are not innocent bystanders but actively participate in the local inflammatory process. Staining for Tac, 4F2 and T9 was completely negative, however. These findings suggest that the local T cell activation may be incomplete and that it is not as extensive as in autoimmune liver diseases.

Expression of Ia and the production of interleukin 1 by peritoneal exudate macrophages activated [formula omitted] by steroids

The in vivo stimulatory potential of estrogens and progestogens on monocytes/macrophages was examined in peritoneal exudate cells from Balb/cBy mice treated with continuous infusions for 15 days. Mice received a daily dose of 6.6 x 10 −10 M of one of the following compounds: prednisone, testosterone, estrogens - 17 β-estradiol and diethylstilbesterol, progestogens - progesterone and ethisterone and the bile acid lithocholic acid. Although total numbers of peritoneal exudate cells and the percentage of macrophages within those populations did not change with any of the treatments, the number of Ia-positive cells did significantly increase with the two estrogens, the two progestogens and the bile acid given the mice. The production of interleukin 1 was also stimulated by the treatment of the animals with the two estrogens, the two progestogens, and the bile acid, but not the glucocorticoid or testosterone. The activation of Ia expression, therefore, correlated with the production of IL-1. Estrogens and progestogens appear to have a marked effect on in vivo activation of monocytes/ macrophages and may relate to differences in females and males in developing immune responses related to the actions of monocytes/macrophages.

Immunohistochemical analysis of the rat central nervous system during experimental allergic encephalomyelitis, with special reference to Ia-positive cells with dendritic morphology.

The rat central nervous system (CNS) during experimental allergic encephalomyelitis (EAE) was analyzed immunohistochemically from the preclinical to recovery stage by using monoclonal antibodies specific for rat T lymphocyte subsets and Ia antigen. Through combination of the avidin-biotin technique and carefully selected fixative, cells with dendritic morphology (DC) and infiltrating mononuclear cells were clearly and intensely demonstrated in the CNS parenchyma during EAE. In normal and complete Freund's adjuvant (CFA)-injected controls, there were no inflammatory foci. Ia (OX3)-positive parenchymal cells were not detected, whereas W3/25 stained DC that were located mainly in the white matter and W3/13 stained axons. At the preclinical stage, 11 days after CNS/CFA sensitization, a few clusters of Ia+ DC were detected in some sections of the spinal cord. The number of Ia+ DC increased as clinical signs developed (P less than 0.001). In rats with a clinical score of 1 or 2, Ia+ DC were mainly located in the perivascular region and closely associated with infiltrating T lymphocytes. However, at moribund state (score 3), Ia+ DC were evenly distributed in gray and white matter on almost all sections of the spinal cord. In recovered rats, the numbers of inflammatory foci and Ia+ DC were less than those in clinical EAE rats (P less than 0.001). Rats without clinical signs throughout the course also contained a few clusters of Ia+ DC. Double immunofluorescent staining with OX3 and anti-glial fibrillary acidic protein (GFAP) antiserum demonstrated that Ia+ DC were negative for GFAP. Their morphology and distribution were similar to those of nucleoside diphosphatase-positive cells, suggesting that Ia+ DC are microglia. In contrast to DC, no astrocytes or endothelial cells express detectable levels of Ia antigen in control and clinical EAE rats. These findings suggest that brain cells other than Ia+ DC may not be involved in the local immune interaction. Ia+ DC may play a significant role in antigen presentation in the CNS with EAE.

Immunoregulatory molecules and IL 2 receptors identified in multiple sclerosis brain.

Frozen brain specimens from eight multiple sclerosis (MS) patients were examined for the presence of leukocytes and their cell products by using a panel of monoclonal antibodies. The results presented here demonstrated that in the MS lesion, there was a marked accumulation of HLA-Dr-positive (Ia-positive) cells. These Ia-positive cells were identified as being glial fibrillary acidic protein positive by using double staining methods. Furthermore, the cells in the MS lesion expressed the interleukin 2 (IL 2) receptor, as identified by the anti-TAC monoclonal antibody. The cells in the region of the plaque also exhibited positive staining with antibodies to IL 1, IL 2, and prostaglandin E. Neither normal brain nor brain specimens from progressive multifocal leukoencephalopathy or Alzheimer's disease showed such patterns of staining. These results suggest that stimulated cells are present in the MS brain, thus implicating an active immune mechanism in the pathogenesis of MS.

Immunohistological analysis of macrophages in the central nervous system of Lewis rats with acute experimental allergic encephalomyelitis

Inflammatory cells were characterized by immunohistochemistry, utilizing monoclonal antibodies specific for T cell subsets, B cells, Ia-positive cells and cells of the mononuclear phagocyte system, in cryostat sections of central nervous system of Lewis rats, sacrificed during the course of actively induced experimental allergic encephalomyelitis. The present study provides interesting information about the presence and distribution of cells of the monocyte-macrophage lineage in this immunologically mediated disease. Using these monoclonal antibodies different subpopulations of macrophages having varying distribution patterns in the central nervous system can be recognized.

Monoclonal antibody analysis of canine hemopoietic cells. Role of Ia-like and Thy-1 antigens in bone marrow engraftment.

The expression of Ia-like (class II MHC) antigens on canine hemopoietic cells was investigated using a cytotoxic murine monoclonal antibody, WM-2, reactive with dog Ia-like antigens. Another monoclonal antibody, WMD-1, reactive with canine Thy-1 antigen, was used as a positive control. Depletion of Ia+ cells from dog bone marrow by complement-mediated lysis with WM-2 antibody failed to inhibit growth of granulocyte-macrophage progenitors (CFUGM) in vitro, while WMD-1 produced complete inhibition of CFUGM. Lethally irradiated dogs receiving bone marrow autografts depleted of Ia-positive cells ex vivo showed initial engraftment, followed by prolonged pancytopenia, and eventual complete recovery of marrow function in the majority of animals. In contrast, dogs receiving autografts treated with WMD-1 and complement all died of marrow failure. We interpret these results as indicating: (1) that Thy-1 antigen is present on hemopoietic stem cells essential for marrow engraftment; and (2) that the expression of Ia antigens on hemopoietic cells is heterogeneous and related to the level of stem cell maturation. While Ia appears to be present on a stem cell population at an earlier stage than CFUGM, as evidenced by the transient phase of graft failure seen in dogs receiving Ia-depleted marrow, the most primitive stem cell, responsible for long-term engraftment, is effectively Ia-negative.

Staphylococcal Protein A Primed Leukocytes Enhance the Autologous Mixed Lymphocyte Reaction

Human peripheral blood mononuclear cells (PBMC) were preincubated for 3 days in medium alone or with various mitogens then washed and irradiated. The preincubated cells then were cultured with autologous T-cells in an autologous mixed lymphocyte reaction (AMLR). Staphylococcal protein A (SPA) pretreatment of PBMC enhanced autologous T-lymphocyte proliferation from 1375 +/- 321 cpm (mean +/- SEM untreated PBMC) to 42,467 +/- 7,985 cpm (SPA primed PBMC) (p less than 0.01). The ability of SPA treated PBMC to enhance the AMLR was not simply a reflection of their proliferation in preculture, as PBMC precultured with phytohemagglutinin and concanavalin A showed greater proliferation than SPA-treated PBMC yet only minimally enhanced the AMLR. Kinetic studies and pre-exposure of PBMC to graded doses of gamma radiation showed that SPA augmentation of the AMLR was mediated by 2 components which differed in kinetics and radiosensitivity. Although incubation of PBMC with SPA did not increase the percentage of cells with detectable surface Ia antigen, SPA did increase the density of Ia in the preincubated cells. Cell separation studies revealed that SPA enhancement of the AMLR was not mediated by T-cells, but was mediated by a non-adherent non-E-rosetting fraction of cells. SPA enhancement of the AMLR was associated with an increased Ia density in the stimulator population but not with an increase in Ia positive cells and was mediated by proliferation-dependent and proliferation-independent mechanisms.

PARTICIPATION OF ANTIGEN PRESENTING CELLS IN GLOMERULONEPHRITIS

The participation of glomerular antigen presenting (Ia positive) cells in the development of glomerulonephritis was analyzed. Antigen-presentation by kidney cells was evident from the assay of ovalbumin-specific T cell proliferation responses by these cells. Experimental serum sickness nephritis was induced in preimmunized rats by the intraperitoneal sensitization of ovalbumin, and the participation of antigen presenting cells (APCs) in the peritoneal cavity and glomeruli was studied. APCs in the peritoneal cavity progressively increased in number following the antigen stimulation, while those in glomeruli were shown to increase in correlation with the development of glomerular hypercellularity. These results imply that the role of APCs in the sensitized area is superior in this model of immune complex nephritis.

Modulation of the population density of identifiable epidermal Langerhans cells associated with enhancement or suppression of cutaneous immune reactivity.

The epidermis on the backs or ears of DBA/2 mice treated for 7 days with a 20% concentration of monobenzyl ether of hydroquinone (MBEH) had a significantly greater population density of ATPase- and Ia-positive cells compared with control mice treated with diluent. There was no decrease or increase in ATPase- or Ia-positive cells at sites distal from the treated tissue. This increase in population density of Langerhans cells was associated with a significant increase in functional afferent immune reactivity measured by allergic contact hypersensitivity. We also found evidence for enhanced efferent immune reactivity. Animals treated on the ears for 7 days with MBEH were sensitized to DNFB on untreated back. MBEH treated ears with more Ia-positive Langerhans cells demonstrated a threefold greater increase in swelling after the DNFB challenge than the control mice. Results of other studies suggest that the afferent and efferent enhanced immune reactivity produced by MBEH are local effects. We postulated that MBEH produced its effects by activating the oxidation of arachidonic acid (AA) to prostaglandins. To test this, we applied AA to mouse skin. AA has a biphasic effect on epidermal Langerhans cells: in low doses it increases their number; in high amounts it decreases the number of identifiable cells with either the Ia or the ATPase technique. An increased population density of identifiable epidermal Langerhans cells induced with AA was correlated with an increase in afferent and efferent immune reactivity. In contrast, reduction of Langerhans cells with larger amounts of AA suppress the afferent and efferent limb of the immune response. DNFB applied to skin with decreased Langerhans cell density from AA induced a state that mimics immune tolerance. The findings are significant because we report the only method to either increase or decrease the population density of Langerhans cells: and to modulate up or down the afferent or efferent limbs of the cutaneous immune response. Our results also suggest that the Langerhans cell may be involved in the efferent limb of the immune efferent response. These effects may be modulated in part by products of AA metabolism.

Dysfunction of Ia-positive antigen-presenting cells in autoimmune mice

Antigen-presenting activity of spleen macrophages for T-cell activation was studied in vitro using autoimmune prone MRL/Mp- lpr/lpr (MRL/l) mice, and their normal counterparts, MRL/ Mp-+/+ (MRL/n) mice. In vitro induction of trinitrophenyl (TNP)-specinc proliferative T-cell response in lymph node lymphocytes from picryl chloride-painted mice was markedly impaired in MRL/l mice. The presenting activity of TNP-hapten by spleen macrophages for proliferative T cells was also impaired in MRL/l mice. The impairment of both T-cell and antigen-presenting macrophage activities was marked in older MRL/l mice, but not in younger ones. The impaired antigen-presenting activity of macrophages was not due to the development of suppressor macrophages. In accordance with the impaired antigen-presenting activity of macrophages the population of Ia-positive cells in spleen macrophages and the production of interleukin 1 by spleen macrophages were also reduced in older MRL/l mice. These results suggest that Ia-positive macrophages are impaired in autoimmune prone mice and that the dysfunction of Ia-positive macrophages plays an important role in the pathogenesis of autoimmune diseases.

Effect of cysteine ethylester hydrochloride (Cystanin) on host defense mechanisms (II): Restorative effects on the suppression of antibody production

The production of hemolytic plaque forming cells (HPFC) in the spleen of BALB/c mice immunized with sheep red blood cells was significantly inhibited by carrageenan treatment (0.3 mg/kg, i.p.; on days -3 and -1). Cysteine ethylester hydrochloride (ethylcysteine) restored the inhibition of the HPFC production by carrageenan treatment in a dose-dependent manner (10-100 mg/kg, p.o.). Ia positive cells (antigen-presenting cells) increased in the spleen adherent cells (SAC) obtained from immunized mice, whereas they decreased in the SAC obtained from carrageenan-treated mice. An increase of Ia positive cells occurred in the SAC of carrageenan-treated mice given ethylcysteine. Ethylcysteine (10-100 mg/kg, p.o.; on days -2 and -1) prevented both the suppression of the HPFC production and the decrease of the number of thymus lymphocytes and peripheral leukocytes induced by cyclophosphamide treatment (30 mg/kg, i.p.; on days -1 and 0). Lyt 1.2 positive cells (helper T cells) decreased in the spleen T cells of cyclophosphamide-treated mice, but increased in the spleen T cells from cyclophosphamide-treated mice give ethylcysteine. On the other hand, Thy 1.2 negative cells (B cells) did not increase in the spleen cells of cyclophosphamide-treated mice with or without ethylcysteine. These results suggest that ethylcysteine restores the immune response in immunosuppressed mice through the functions of macrophages and/or helper T cells.

誘導マクロファージの機能に関する研究 : I 報 マクロファージの Ia 抗原の測定法について

In induced peritoneal macrophages, the quantity of Ia antigen was measured by monitoring absorbance using enzyme-linked immunosorbent assay (ELISA). The number of Ia positive cells were counted simultaneously under microscope. Results obtained were as follows : 1. An optimum time for enzyme reaction in ELISA was 40 min using 4×10^4 cells as a sample. 2. Absorbance in ELISA correlated closely with the number of Ia positive cells. 3. It was shown that an increase in absorbance was brought about by an increase in the number of Ia positive cells and not by an increase in Ia quantity in the surface each cell. These results showed that ELISA was a useful method for the study of quantifying cell surface antigens like Ia antigen.

Self-tolerance to host and donor following HLA-mismatched bone marrow transplantation.

The transplantation of T cell-depleted HLA-haploidentical bone marrow can correct the severe combined immunodeficiency disease (SCID) caused by the inherited absence of T lymphocytes. Despite a different environment, no severe graft-vs.-host reaction occurred and engrafted T lymphocytes became functional. We have studied tolerance of engrafted T lymphocytes to donor and host HLA antigens in four SCID patients who have been transplanted with bone marrow from one of their HLA-haploidentical parents. Graft-vs.-host reaction was prevented by T cell depletion of infused bone marrow using E rosetting and by in vivo administration of cyclosporine A. Subsequent to bone marrow transplantation (BMT), the engrafted T lymphocytes were shown to be unresponsive in vitro towards host cells collected prior to BMT. Generally, this tolerance could not be explained by a suppressive mechanism. Nevertheless, in one patient suppressive cells were found transiently. In contrast to the early appearance of a tolerance towards host, a reactivity of engrafted donor cells towards donor was always observed within the first 300 days post-grafting. This autoreactivity was mediated by T cells of donor origin and its targets were HLA class II molecules (at least HLA-DR and DQ). The progressive disappearance of this autoreactivity was correlated with the engraftment of Ia-positive cells (monocytes plus B lymphocytes) of donor origin and the achievement of complete immunological reconstitution. In the patient showing the strongest autoreactivity, a donor-specific T cell line has been grown which was shown to specifically inhibit the proliferative response of donor lymphocytes. Concomittantly, the immunological reconstitution remains poor in this patient. These data suggest that tolerance to HLA class II molecules is dependent on the presence of the relevant HLA class II molecule-expressing cells allowing the elimination or the suppression of T lymphocytes specifically directed at these molecules.

The distribution of Ia antigen in the lesions of rat acute experimental allergic encephalomyelitis.

Ia antigen, encoded within the major histocompatibility complex, plays an important role in the activation of T lymphocytes. Since experimental allergic encephalitis is an essentially T cell-mediated disease, Ia antigen in the central nervous system (CNS) may be pathogenetically relevant. The occurrence of Ia antigen in the CNS of normal rats and of rats with experimental allergic encephalitis was studied by light and electron microscope immunocytochemistry using the monoclonal anti-Ia antibodies Ox 4 and Ox 6. In normal, unsensitized animals a district population of stellate cells in the meninges and some perivascular mononuclear cells in the nervous tissue carried Ia antigen. In rats with experimental allergic encephalitis a dramatic increase of Ia-positive cells was found. In addition to the positive cells found in normal animals, monocytes, macrophages and many lymphocytes in the meningeal perivascular and parenchymal inflammatory infiltrates as well as "activated microglia" stained for Ia antigen. We did not find evidence for Ia expression on endothelial cells, astrocytes or other components of the CNS in either normal or diseased rats.

The suppressive effect of tape-stripping treatment of guinea-pig skin on the induction of contact sensitivity by intradermal injection of haptenated epidermal cells.

Contact sensitivity (CS) to 2,4-dinitrochlorobenzene (DNCB) was produced in inbred JY1-strain guinea pigs by the intradermal injection of epidermal cells (ECs) prepared from DNCB-painted skin (DNP-ECs). When the site of DNP-EC-induced CS was pretreated by tape stripping, the rate and intensity of the challenge reactions to DNCB were diminished. The ability of DNP-ECs to induce CS returned to normal when normal peritoneal macrophages together with DNP-ECs were administered into the stripped skin. Normal ECs had a similar effect. Using either anti-Ia antiserum and complement or allogeneic ECs (strains 2 and 13), Ia-positive cells among the ECs (presumably Langerhans cells) were found to be essential for the recovery of CS. Tape-stripping treatment also resulted in the development of immunological tolerance, as assessed by subsequent painting with a sensitizing dose of DNCB. These findings suggest that the immunological function of the mononuclear-phagocyte system in the dermis may be impaired when the epidermal surface is markedly disturbed by tape-stripping treatment.

The in vitro regulation of human thyrocyte HLA-DR antigen expression.

The ability of endocrine organs to express human immune response-associated antigens (Ia), such as HLA-DR, is a subject of intense current interest. In this study, the effects of various potential modulators of thyroid follicular cell HLA-DR expression were examined using in vitro cultures. A culture supernatant containing T-cell-derived lymphokines caused DR antigen expression on 13-18% of thyroid cells; more consistent effects were produced by recombinant gamma-interferon, which led to 46-100% of the thyroid cells becoming HLA-DR positive after 3 days in culture. This effect was both time and concentration dependent and occurred in thyroid cells derived from patients with Graves' disease (n = 7) and Hashimoto's thyroiditis (n = 2) as well as from three subjects with no autoimmune thyroid disease. Thyroid cells stained with the monoclonal antibodies 4F2 and 5E9, which recognize cell activation antigens, regardless of whether they were treated with gamma-interferon. The lectin phytohemagglutinin also induced HLA-DR antigen expression (21-91% of cells positive). This response was dependent on T cell contamination of thyroid cell suspensions, since the effect was inhibited by cyclosporin A. HLA-DQ antigen expression, identified by the Leu-10 monoclonal antibody, was also induced on thyroid cells by gamma-interferon and phytohemagglutinin. In contrast, neither recombinant alpha-interferon nor interleukin-2 induced HLA-DR antigens. Irradiation reduced the response of thyroid cells to gamma-interferon, but two of the known inhibitors of macrophage Ia expression, prostaglandin E2 and (Bu)2cAMP, did not affect gamma-interferon-induced thyroid cell HLA-DR expression. We were unable to detect interleukin-1 production by thyroid cells. These results suggest that 1) under normal circumstances, thyroid cells are 4F2 and 5E9 positive, but are incapable of expressing Ia antigens and, thus, of activating T cells to initiate autoimmune thyroiditis; and 2) once activated, for example by a virus, T cells could release gamma-interferon and induce thyroid cell HLA-DR and -DQ antigen expression; these Ia-positive thyroid cells could then have a role in maintaining or enhancing the autoimmune response.

Pattern of infection and selective loss of Ia positive cells in suckling and adult mice inoculated with lactic dehydrogenase virus.

Lactic dehydrogenase virus (LDV) infected cells were localised in cryostat sections of infected adult and suckling mice by fluorescent antibody staining. In almost every organ except brain and spinal cord, LDV infected cells were present in interstitial connective tissue, including dermis and submucosa of gastrointestinal and urinogenital tracts. They were also present in liver sinusoids and red pulp of spleen. The tissue distribution, shape, and fluorescence staining pattern of infected cells were similar in adult and suckling mice. The reactivity with monoclonal antibody to mouse macrophages (F4/80) and to Ia antigens indicated that infected cells were Ia antigen positive macrophages, and this was confirmed in double labelling experiments. Peak numbers of LDV infected cells were seen in every organ 12-18 hours post infection (p.i.), disappearing rapidly thereafter until at 48 hours p.i. they could no longer be detected. At the same time Ia positive cells fell to undetectable levels, although macrophages were still present in reduced numbers. At 7 days p.i. the total number of macrophages in sections had returned to normal, but the number of Ia positive cells remained at low levels. Macrophages recovered from peritoneal cavity and spleen of intraperitoneally infected mice were also studied. Ia positive cells had virtually disappeared from peritoneal cavity at 24-48 hours, and from spleen at 24-72 hours. Three weeks after infection the proportion of Ia positive cells was still low when compared with that of normal mice suggesting selective loss of these cells.

Detection of Cell‐Mediated Lympholysis Determinants Encoded by the I‐A and I‐E Genes on Ultraviolet‐Irradiated Spleen Cells

We have analysed the effects of ultraviolet (UV) irradiation on the expression, by normal splenic cell populations, of cell-mediated lympholysis determinants encoded by the I-A and I-E genes of the H-2 complex. Cytotoxic T lymphocytes specific for class II antigens could be generated from unprimed responder mice when stimulated with UV-treated cells from I-A-and/or I-E-congenic mice in the presence of interleukin 2-containing medium. A cytotoxic T-cell line specific for I-Ek antigens was obtained and maintained upon stimulation with UV-irradiated spleen cells. Secondary Ia-specific responses were obtained with UV-irradiated stimulators in the absence of added soluble factors. The results show that the failure of UV-irradiated cells to stimulate primary cytotoxic T-cell responses is not due to a selective destruction of class II antigens but rather to the inactivation of a still undefined function of Ia-positive cells, other than interleukin-1 production, involved in the establishment of high-affinity interactions between the responding T helper cells and the stimulator cells.

Ia antigens and Fc receptors of mouse peritoneal macrophages as determinants of susceptibility to lactic dehydrogenase virus.

The relationship between susceptibility of mouse peritoneal macrophages to lactic dehydrogenase-elevating virus (LDV) infection and expression of I region-coded antigens (Ia) on these cells was investigated. The proportion of Ia-positive cells in resident peritoneal macrophages from adult and suckling mice were 4 to 10% and 50 to 70% respectively. Approximately the same percentage of the cells were susceptible to LDV, as detected by fluorescent antibody staining. In adult mice, double-labelling experiments showed that most of the Ia-positive cells were LDV-infected. When the cells were cultured for more than 24 h in vitro, Ia-positive cells rapidly disappeared and the culture became resistant to LDV. Removal of Ia-positive cells by treatment with anti-Ia plus complement or enrichment using an anti-Ia-coated Petri dish simultaneously removed or enriched for LDV-susceptible cells. Treatment of cells with trypsin (1 mg/ml) removed their I-A and I-E antigens and simultaneously abolished susceptibility for LDV. When LDV was preincubated with subneutralizing amounts of antibody, infectivity for macrophages was enhanced and the proportion of LDV-infected cells was higher than that of Ia-positive cells. This suggests that Fc receptors on macrophages can act as receptors for LDV coated with antiviral IgG.